Circuitry and process for testing non-intermittent signal generators

ABSTRACT

In the case of a multichannel monitoring device for monitoring system operating states, a test input (45a, 45b) is provided per channel (2, 3) on an evaluation and monitoring circuit for the purpose of testing the supply leads (7a, 7b) to the signalling units (1a, 1b) for freedom from damage. Either no filters or filters (47a, 47b) with a short settling time are provided in the supply lead to this test input (45a, 45b). As a result, testing requires only a very short time, so that the evaluation of the signalling units (1a, 1b) is interrupted only for a very short time. The time is so short that no dangerous system states can occur.

BACKGROUND OF THE INVENTION

1. Field of the Invention

With controllers, it is necessary to distinguish between intermittentand non-intermittent sensors or signalling units. In the case ofintermittent signalling units or sensors, a functional test can beachieved with the aid of a plausibility test of the signal coming fromthese sensors or signalling units. This is not so, however, with theso-called non-intermittent sensors or signalling units, which supply anunchanged continuous signal over a long period of time. A simple exampleof such a sensor or signalling unit is a door contact switch, which inthe cases both when the door is open and when it is closed emits acontinuous signal as long as the operating state of the door does notchange.

On the other hand, however, it is very important even in the very caseof such signalling devices to determine whether they are still operatingsatisfactorily or whether damage has occurred and, possibly, anoperating state is being signalled which no longer corresponds to thetrue conditions. An example in the case of a door contact switch wouldbe the short circuiting between the two door contact switches in thecase of a two-channel monitoring circuit. The consequence would be thatonly one door contact switch has to be actuated, while the position ofthe other door contact switch is no longer important. From the point ofview of the evaluation circuit, both switches would be actuated aspreviously.

A further difficulty in the functional testing of the signalling unitsis caused by the amplitude or frequency filters normally required, thesignal units being necessary satisfactorily to debounce and remove theinterference from the signal coming from the signalling unit, in orderto exclude damage to and maloperation of the inputs of the downstreamevaluation circuit of the controller. These filters can have asubstantial settling time. The signal at the output of the line whichconnects the signalling unit to the relevant evaluation circuit wouldhave to be of a length corresponding to the filter settling time. Thisperiod is in conflict with the requirement to keep the testing time forthe signalling unit as short as at all possible so that no dangerousoperating states occur. To be precise, each extension of the testingtime means that as long as the test is running the monitoring circuitdoes not detect if the signalling units signal a dangerous operatingstate of the monitored system. In addition, with multichannel monitoringdevices the testing time is lengthened in accordance with the number ofthe channels, since all the signalling units monitoring the same stationin a system must be tested simultaneously.

2. Description of the Related Art

By contrast, it is easier to handle transmission lines which areinterconnected galvanically, transmit AC signals and, in addition, areconnected at the end via transformers to the line drivers and linereceivers. As shown in WO 94/00771, these lines can be tested with theaid of DC signals while the AC voltage transmission is proceedingsimultaneously. Provided for this purpose at both ends of the line ineach case is a balun which is connected at one end to the transmissionline and at the other end to a symmetry transformer. Connected for thepurpose of electrical interruption in the connecting line between thesymmetry transformer and the balun is a capacitor which passes theuseful AC voltage signal and to which the DC voltage used for testing isapplied for testing purposes. This DC voltage can be tested for polarityand amplitude at the capacitor at the other end of the line. It isdetermined in this way whether the line is connected satisfactorily,interrupted or else has a cross.

Another possibility for testing a multi-core cable is described in thearticle "Timer IC's and LEDs for Cable tester", which appeared in"Electronics" 1973, pages 115 ff. A ring counter is connected to one endof a cable, while the other cable end is connected to one base of atransistor complementary stage. The other base is connected directly tothe relevant output of the ring counter, which has a stage for each corein the cable. Depending on the routing conditions in the cable, there isa different current direction through a double LED connected in parallelwith the transistors.

However, it is not possible using this test device to test the cable inthe state of use. Rather, it has to be taken out of the transmissionlink.

OBJECTS AND SUMMARY OF THE INVENTION

Starting from here, it is the object of the invention to provide amonitoring device which can be used to perform quick tests of thesignalling units or sensors belonging to the monitoring device.Moreover, it is the object of the invention to specify a correspondingmethod.

On the basis of the special configuration of the monitoring device, thesignalling units and their supply leads are tested, as it were, from therear or by using a short settling time, as a result of which thesettling times of the filters contained at the signal outputs for normaloperation can be largely eliminated. For example, with the aid of thenovel circuit arrangement except for the power supply input of onesignalling unit, all the remaining signalling units are loaded at thepower supply input with a defined impedance, after which the voltagepresent at these loaded power supply inputs is tested. This circuitstate is carried out cyclically for all the signalling units, in orderto test whether there is a short circuit to the supply voltage. In sucha case, parasitic voltages are formed from that signalling unit whichreceives a supply voltage to a signalling unit which is loaded with adefined impedance. This formation of parasitic voltages can be detectedat once. Since the testing is performed on the feed side of thesignalling units and not on the side of the signal output, the settlingtimes which are present between the output of the signalling unit andthe normal input of the downstream evaluation circuit are largelyeliminated. It is possible to use substantially quicker filters whichsuppress only interference signals but are not designed to suppresspossible longer bounce times.

On the other hand, the novel circuit arrangement also permits thedetection of a range of ground faults situated between the signallingunit and the voltage source driving the respective signalling unit.

If, for example, with a signalling unit or sensor which is open, damageoccurs to the supply cable with the consequence of a short circuit toearth, this fault can be detected at once. Applying the supply voltageto this defective cable and testing the input voltage lead to diagnosisof the fault because the supply voltage does not rise in the expectedway. Instead, it is loaded by the short circuit, and this leads, incombination with the internal impedance of the voltage source or themeans for varying the voltage, to a corresponding voltage drop.

In a simple embodiment of the circuit arrangement, the means for varyingthe voltage and the voltage sources are identical, that is to say use ismade of a voltage source whose output voltage can be switched over. Sucha voltage source whose output voltage can be switched over is, forexample, a push-pull circuit comprising two transistors whose seriescircuit is connected between the supply voltage and the circuit ground.Depending on which of the two transistors is driven, the voltage sourceeither supplies the normal supply voltage, or it grounds its voltageoutput with a defined internal impedance. The detection of faults in thesupply leads and the signalling units is simplified if the internalimpedance with which the signalling units are fed depends on the currentflow direction. As a result, the settling times when switching over tothe normal supply voltage are shortened while, on the other hand, theline can be discharged without short circuiting parasitic voltagesdirectly to ground.

Dangerous parasitic voltages can be avoided if the evaluation circuit iscoupled to the power supply input of the signalling unit or the outputof the associated voltage source via an optocoupler.

If the signalling unit also permits a detectable current in the reversedirection from the signal output to the power supply input, it is alsopossible to detect cable damage which is situated between the signallingunit and the normal input of the associated evaluation circuit. This isalways ensured in any case where simple switches are concerned. However,should the signalling unit not have this ability of itself, in somecircumstances the use of an appropriately poled diode suffices toprovide this function.

For the rest, developments of the invention are the subject-matter ofthe subclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the subject-matter of the invention arerepresented in the drawings, in which:

FIG. 1 shows a block diagram of the novel monitoring device, thesignalling units being tested from the feed side, and

FIG. 2 shows a block diagram of the novel monitoring device, thesignalling units being tested from the signal output.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An excerpt from a machine controller is illustrated as a block diagramin FIG. 1; specifically that region which serves to monitor a specificstation of the controlled system with the aid of two signalling units1a, 1b is shown by way of example. The monitored station is, forexample, the access door in a protective fence behind which a machine islocated. The monitoring device shown contains a channel 2 and a channel3, which are of identical design. For this reason, the same referencesymbols are used for the components of the two channels 2 and 3, beingsupplemented by "a" or "b" for distinguishing purposes. The followingexplanation of the design relates initially to channel 2.

Channel 2 contains a voltage source 4a to whose current output 5a thesignalling unit 1a is connected via a line 6a. A further line 7a leadsfrom the signalling unit 1a to a first input filter 8a, which isconnected on the output side to a further filter 11a via an optocoupler9a.

The voltage source 4a comprises two NPN transistors 12a and 13aconnected in a cascade, the transistor 13a being connected with itsemitter to a circuit ground 14a. A connection leads from the collectorof the transistor 13a to the emitter of the transistor 12a, whosecollector is connected to a positive supply voltage 15a. The two basesof the transistors 12a and 13a are connected to a phase-reversal stage16a which has a control input 17a. A resistor 18a leads to the currentoutput 5a from the collector of the transistor 13a or the emitter of thetransistor 12a, there being connected in parallel with the resistor 18aa diode 19a whose cathode is likewise connected to the current output5a.

With the aid of this voltage source 4a, it is either possible via anappropriate binary control signal at the input 17a for the currentoutput 5a to be grounded to the circuit ground 14a via the resistor 18aand transistor 13a, or in another state of the signal at the controlinput 17a the transistor 13a is blocked and the transistor 12a is turnedon, so that the control output 5a receives voltage from the power supplyline 15a via the resistor 18a.

In the case shown, the signalling unit 1a is a simple switch with apower supply input 21a and a signal output 22a. The current input 21a isconnected via the line 6a already mentioned to the current output 5a.Like the line 7a, the line 6a is of substantial length. The lengthdepends on the distance between the signalling unit 1a and the site atwhich the monitoring circuit or the machine controller is located. Thelengths of the lines are normally a few meters to a few tens of meters.Appreciable parasitic interference must be expected. The line 7a istherefore also connected to an input terminal 23a of the filter 8a,which comprises an RC low-pass filter 24a and an amplitude filter 25a.The RC low-pass filter 24a contains an ohmic resistor 26a which isconnected to the input terminal 23a and whose other end is connected tothe circuit ground 14a via a capacitor 27a. A further limiting resistor28a leads from the connecting point between the resistor 26a and thecapacitor 27a to a diode 29a whose anode is connected to the circuitground 14a. The optocoupler 9a is connected in parallel with the diode29a with its input terminals 31a and 32a. The result of this is toachieve electrical isolation with respect to the circuit following onthe right in the circuit diagram.

The optocoupler 9a has two output terminals 33a and 34a, the outputterminal 34a leading to a further circuit ground 35a, while the outputterminal 33a is connected to a supply voltage V_(cc) via load resistor36a. The output signal, which passes into an input 37a of a furtherfilter 11a, is tapped at the connecting point between the resistor 36aand the output terminal 33a. This filter 11a has a signal output 38a anda ground terminal 39a.

The channel 3 already mentioned is designed with the same components inan identical circuit. Present in common for both channels 2 and 3 is atest control circuit 41 with two control outputs 42a and 42b which areconnected to the control inputs 17a and 17b, respectively, of thephase-reversal stage 16a and 16b, respectively.

Furthermore, for both channels 2 and 3 there is a common control andevaluation circuit 43 having two normal inputs 44a and 44b, which areinterrogated in normal operation, and having two test inputs 45a and45b, which are interrogated in the test mode described further below.The normal input 44a is connected to the output 38a of the filter 11a,while the test input 45a leads to an output 46a of a filter 47a whoseinput 48a is connected to an output terminal 49a of an optocoupler 51a.A further output terminal 52a of the optocoupler 51a is connected to thecircuit ground 35a, while the output terminal 49a is connected to thesecond supply voltage V_(CC) via a load resistor 53a.

The input side of the optocoupler 51a with input terminals 54a and 55ais connected to the current output 5a and, via a resistor 56a, to thesupply voltage 15a.

The same design applies in turn analogously for channel 3.

Although the circuit diagram is shown only with two channels 2 and 3, itis also possible to use three parallel-connected channels in order toenhance the safety even further. This third channel would then in turnhave the same design as channel 2, and the circuits 41 and 43 common tothe two channels would respectively have a further third output or afurther normal input and a further test input.

With regard to the mechanical design, it is further important to mentionthat the two voltage sources 4a and 4b are accommodated together withthe test control circuit 41 in the spatial vicinity of the filters 8a,11a, 47a and the evaluation and control circuit 43, for example in thesame housing. The current output 5a and the input 23a are consequentlyalso directly adjacent, and the two lines 6a and 7a are a two-wireconnection. The block diagram according to FIG. 1 therefore does notreproduce the true spatial conditions, but only the electricalconditions.

Finally, it remains to be stated that the test control circuit 41 andthe evaluation and control circuit 43 are realized in microprocessors ormicrocomputers, or represent program parts in a single microcomputer ormicroprocessor. The way in which these circuits 41 and 43 are designedin terms of hardware depends on the respective application and,moreover, has no repercussions for the invention.

The mode of operation of the monitoring device thus described is asfollows:

Normal Operation

In normal operation, the evaluation and control circuit 43 evaluates theoperating state of the two signalling units 1a and 1b, that is to saythe sensor, designed as a switch, with the aid of which, as mentionedabove, for example, the position of a protective door or an access dooris monitored. If the danger area is closed, the two switches 1a and 1bare likewise closed, that is to say closed-circuit signalling units areconcerned here.

In order to realize this function, the evaluation and control circuit 43controls the test control circuit 41 in such a way that the latter emitsat its outputs 42a and 42b the binary signals with a state intended toturn on the transistor 12a or 12b via the phase-reversal stages 16a and16b, respectively, while the respective other transistor 13a or 13b isswitched into the blocking state. The current output 5a is thus at thepotential of the supply voltage 15a.

This current/voltage signal supplied from the voltage source 4a passesvia the line 6a to the signalling unit 1a and on from there via the line7a into the input 23a of the filter 8a. With the aid of the filter 8a,or in the other channel 3 with the aid of the filter 8b, the voltagesignal which is passed into the input 23a or 23b, respectively, is freedfrom radio-frequency disturbing pulses and impermissible voltage peaksbefore it is fed into the optocoupler 9a. From here, the signal passesthrough a further filter 11a, likewise with a low-pass characteristic,and possibly after pulse shaping into the normal input 44a or 44b in thecase of the other channel 3. The filter 11a or 11b is intended tosuppress the remaining interference, and to debounce the signal. Thefilters 11a and 11b have a correspondingly low cut-off frequency.

The evaluation circuit 43 tests continuously in normal operation whetherthe two channels 2 and 3 signal the same positions of the switches 1aand 1b in the two channels 2 and 3 inside a stipulated time window. Ifone of the switches 1a or 1b fails, or if the controlling voltage source4a or 4b fails, inequality arises between the signals at the normalinputs 44a and 44b, and this causes the evaluation and control circuit43 to detect a fault, which entails immediate stoppage of the entiresystem.

The filter 11a simultaneously has the function of suppressing thedisturbing pulses inevitably occurring in mechanical switches, whichoccur when the switch 1a or 1b is closed and bounces in the process. Theconsequence of this is that two filters 8a and 11a and the correspondingfilters in channel 3 have a comparatively long settling time. Thesettling time is in the region of around 1 to 5 ms, so that because ofthis long settling time functional tests can either not run at all, orcan run only very seldom, via this chain of the filters 8a and 11a,because the state of the two switches 1a and 1b necessarily cannot beinterrogated during the test. A special test mode is therefore providedin which the evaluation and control circuit ignores the signals arrivingat the normal inputs 44a and 44b during the test phase.

Test Phase

When doors or comparable parts of a system are monitored, it is naturalthat the appropriate switches, that is to say the two switches 1a and1b, are actuated only extremely seldom in comparison and do not changetheir circuit state for a long time. Faults which occur in the supplyleads 6a and 7a or 6b and 7b, respectively, remain undetected for a longtime correspondingly if no special arrangements are made. The evaluationand control circuit 43 periodically switches on the test mode in orderto be able to detect damage early. In the test mode, the evaluation andcontrol circuit 43 causes the test control circuit 41, for example inchannel 2, to turn off the transistor 12a and to turn on the transistor13a. If there is no short circuit between the lines 6a and 7a, on theone hand, and the lines 6b and 7b, on the other hand, the voltage at thecurrent output 5a, which is now grounded with the resistor of the ohmicresistor 18a, must vanish within a stipulated time. There is acorresponding increase in the voltage drop on the input side of theoptocoupler 51a, which relays this change in voltage into the test input45a via the filter 47a.

The filter 47a is necessary only if relatively high radio-frequencyinterference which is to be kept away from the test input 45a is to beexpected. In any case, the filter 47a has a substantially higher cut-offfrequency and therefore settles more quickly by at least a factor of 10than the series circuit comprising the filters 8a and 11a. Inparticular, the filter 47a does not need to take account of the possiblylong-lasting bounce times of the mechanical switches 1a and 1b. Theevaluation circuit therefore obtains very quickly at the test input 45aknowledge of whether in accordance with a defective electricalconnection the voltage at the current output 5a vanishes at once.

If, as a consequence of pinching of the cable, there is a crossconnection, for example from the line 6a to the line 6b, the voltage ofthe current output 5b is transferred via this short circuit to thecurrent output 5a as a parasitic voltage, that is to say although thetransistor 13a is conducting, a relatively high voltage is produced atthe current output 5a and its presence is signalled within a very shorttime via the optocoupler 51a to the test input 45a of the evaluation andcontrol circuit 43. The evaluation and control circuit 43 thus detectsthe error, and is efficiently able to shut down the system which is nolonger operating properly.

After this test cycle has been run through, the test control circuit 41ensures that the transistor 12a is turned on again after the transistor13a was blocked. For channel 3, the transistor 13b is likewise turned onand the transistor 12b turned off, in order also to have the same testrun in channel 3. At the same time, the functional ability andcontrollability of the other voltage source 4b is also tested, and shortcircuits can be detected with a diode characteristic. The reciprocalthorough testing of each individual channel is particularly necessarywhenever at least a third channel is further present, because then shortcircuits can occur between channel 2 and the further channel or channel3 and the further channel, and these cannot be detected merely bytesting channel 2.

The general appearance of the test algorithm for n channels is such thatduring the test phase all n voltage sources 4 are firstly switched offvia the test control circuit 41 and then one voltage source 4 afteranother is respectively switched on individually sequentially, that isto say in a 1-of-n procedure. In the nondamaged state, no voltages maythen occur at the other current outputs 5.

Because of the extremely short settling time of the filter 47a, thistest can be run very quickly. Because the line 6a and 7a is testedvirtually from the rear, this test circumvents the filter circuit 8a and11a.

Furthermore, it is possible using the novel arrangement also to detectshort circuits between the line 6a and the circuit ground 14a, or 6b andthe circuit ground 14b, with switches 1a and 1b open. If such a shortcircuit is present, the voltage at the current output 5a collapses,although the relevant voltage source 4a is switched into a state inwhich it would supply current per se. The voltage state at the currentoutput 5a is immediately transmitted in turn by the optocoupler 51a tothe relevant test input 45a so that the evaluation and control circuit43 can take the necessary measures.

If, as shown, the signalling unit used is not a single switch which canalso conduct the current reciprocally, it is possible to connect anappropriately poled diode in parallel with the signalling unit so thateven damage to the line 7a can be detected from the current output 5a.

FIG. 2 shows an alternative embodiment which is likewise suitable forquick testing and which differs from FIG. 1 in that instead of theoptocoupler 51, use is made of filters 61a and 61b whose input 62a isdirectly connected to the output 33a of the optocoupler 9a. On theoutput side, the filter 61a is connected to the alreadymentioned testinput 45a of the evaluation and control circuit 43. It is possible inthis way to circumvent the long settling time of filter 11a, which insome circumstances is relatively long. Otherwise, the test crossesbetween lines 6a, 7a, on the one hand, and lines 6b, 7b, on the otherhand, run in the same way as is explained above in conjunction withFIG. 1. The sole difference resides in the fact that no test is possiblewith contacts 1a and 1b open, because no current can flow.

Finally, it may be mentioned that filters 11a, 47a and 61a can naturallyalso be realized as software filters in the microprocessor-basedevaluation and control circuit 43. Inputs 44 and 45 then correspond tocorresponding switches in the program, depending on whether the filterfunction is to be performed with a large or small time constant.

It is important in any case that the connection between current output5a and input 23a is tested for the purpose of testing freedom fromdamage with other filters or filter constants, so that monitoring by thesignalling units 1a and 1b is interrupted only for a very short time.The time must be so short that no dangerous system states can occur.

In the case of a multichannel monitoring device for monitoring systemoperating states, a further test input is provided per channel on theevaluation and monitoring circuit for the purpose of testing the supplyleads to the signalling units for freedom from damage. Either no filtersor filters with a short settling time are provided in the supply lead tothis test input. As a result, testing requires only a very short time,so that the evaluation of the signalling units is interrupted only for avery short time. The time is so short that no dangerous system statescan occur.

What is claimed is:
 1. A device for monitoring a machine to facilitatesafe operation of the machine, the monitoring device comprising:at leasttwo channels for monitoring system operating states, each channelhaving,a voltage source including a current output which is decoupledfrom said current outputs of said other voltage sources of said otherchannels, a signalling unit having a power supply input coupled to saidcurrent output via a first line, and a signal output, and means forvarying the voltage fed into the first line; a test control circuitcommon to all said channels, said control circuit having a plurality ofcontrol outputs, each one of the control outputs corresponding to adifferent one of the channels, said control circuit controlling eachsaid varying means such that, while no test cycle is running, all thesignalling units are supplied with voltage, and during a test cycle,either (1) each signalling unit is sequentially supplied with voltage inan intermittent temporary fashion, such that only one of the signallingunits is supplied with voltage at a time, or (2) only one of thesignalling units is not supplied with voltage at any one time, such thatduring the test cycle each signalling unit sequentially receives novoltage in an intermittent temporary fashion, or (3) the cycles (1) and(2) are performed sequentially or in an interleaved fashion; and anevaluation and control circuit having, per signalling unit, at least onenormal input and at least one test input, the normal inputs generallycontinuously evaluating the operating state of the signalling unitsduring normal operation, and the test inputs each receiving a signalwhich is derived from a voltage signal which is present at the powersupply input of a corresponding one of the signalling units.
 2. Themonitoring device according to claim 1, wherein the test inputs of theevaluation and control circuit are connected to corresponding ones ofthe current outputs.
 3. The monitoring device according to claim 1,wherein each said varying means, in one operating state, relays thevoltage of the corresponding voltage source essentially unchanged to thecorresponding signalling unit.
 4. The monitoring device according toclaim 1, wherein each said varying means, in one operating state, loadsthe power supply input of a corresponding one of the signalling unitswith a predetermined impedance.
 5. The monitoring device according toclaim 1, wherein the voltage sources are connected to a common powersupply.
 6. The monitoring device according to claim 1, wherein each saidvarying means has an internal impedance which depends on a direction ofthe current flow.
 7. The monitoring device according to claim 1, whereineach said varying means forms the corresponding voltage source.
 8. Themonitoring device according to claim 1, wherein each said varying meanshas first and second electronic switches which are connected in apush-pull fashion, said first electronic switch being connected to saidcommon power supply and said second electronic switch being connected toa circuit ground.
 9. The monitoring device according to claim 1, whereineach said signalling unit is connected via one of said second lines to acorresponding one of the normal inputs of said evaluation and controlcircuit.
 10. The monitoring device according to claim 1, wherein eachsaid signalling unit is conductive in the reverse direction from saidcorresponding signal output to said corresponding power supply input.11. The monitoring device according to claim 1, wherein each test inputof the evaluation and control circuit is connected to a correspondingone of a plurality of second lines which connect a corresponding one ofthe signalling units to a corresponding one of the normal inputs. 12.The monitoring device according to claim 11, wherein either the voltagesource or said varying means of one of said channels has a predeterminedinternal impedance which is larger at least by a factor of 1.5 than acircuit impedance, said circuit impedance defined as the impedancemeasured between the corresponding power supply input and thecorresponding signal output plus the resistance of corresponding ones ofsaid first and second connecting lines.
 13. The monitoring deviceaccording to claim 1, further including a plurality of normal filterseach one being electronically connected upstream of a corresponding oneof the normal inputs, and a plurality of optocouplers each one beingconnected upstream of either a corresponding one of said normal inputsor a corresponding one of said test inputs.
 14. The monitoring deviceaccording to claim 13, further including a plurality of test filters,each test filter being connected upstream of a corresponding one of saidtest inputs of said evaluation and control circuit.
 15. The monitoringdevice according to claim 14, wherein the settling time of said testfilters is shorter than the settling time of said normal filters. 16.The monitoring device according to claim 14, wherein at least some ofsaid test filters and said normal filters are implemented usingsoftware.
 17. A method for testing a device for monitoring a machine tofacilitate safe operation of the machine, the device including at leasttwo channels each having a signalling unit with a power supply input anda signal output, and a voltage source which is decoupled from thevoltage sources of the remaining channels, the method comprising thesteps of:(a) while no test cycle is running supplying all the signallingunits with voltage so as to allow normal operation of the monitoringdevice; and (b) during a number of test cycles corresponding to thenumber of channels either,supplying only one of the signalling unitswith voltage during each test cycle such that the method, during thetest cycles, supplies each signalling unit with voltage sequentially andintermittently, or not supplying only one of the signalling units withvoltage during a test cycle, while supplying the remaining signallingunits with voltage, such that each signalling unit sequentially receivesno voltage in an intermittent fashion during the test cycles, orperforming said supplying and said not supplying steps sequentially orin an interleaved fashion.
 18. The method of claim 17, wherein thevoltage is tested at the power supply input of one of the signallingunits in which, during the test cycle, the supply voltage is connectedin a state other than the normal operating state of that signallingunit.
 19. The method of claim 17, wherein the voltage is tested at thepower supply input of one of the signalling units in which, during thetest cycle, the supply voltage is connected in a state which correspondsto the normal operating state of that signalling unit.
 20. A device formonitoring a machine to facilitate safe operation of the machine, themonitoring device comprising:at least two channels for monitoring systemoperating states, each channel having,a voltage source including acurrent output which is decoupled from said current outputs of saidother voltage sources of said other channels, and a signalling unithaving a power supply input coupled to said current output via a firstline, and a signal output; a test control circuit common to all saidchannels, said control circuit controlling the voltage applied to eachsaid signalling unit such that,while no test cycle is running, all thesignalling units are supplied with voltage, and during a test cycle,either (1) each signalling unit is sequentially supplied with voltage inan intermittent temporary fashion, such that only one of the signallingunits is supplied with voltage at a time, or (2) only one of thesignalling units is not supplied with voltage at any one time, such thatduring the test cycle each signalling unit sequentially receives novoltage in an intermittent temporary fashion, or (3) the cycles (1) and(2) are performed sequentially or in an interleaved fashion; and anevaluation and control circuit having, per signalling unit, at least onetest input, the test inputs each receiving a signal which is derivedfrom a voltage signal which is present at the power supply input of acorresponding one of the signalling units.